Add StorageBuffer.set(index, value) for single-element GPU updates

[aashu2006]

Resolves #8749

Changes

I've added a set(index, value) method to p5.StorageBuffer that updates a single element in a buffer without rewriting the whole thing.

Before this, updating one particle out of thousands required a full GPU -> CPU read, a JS edit, then a full CPU -> GPU write back. Now you can do:

// float buffer
buf.set(2, 9.5);

// struct buffer
particles.set(42, { position: createVector(0, 0), velocity: createVector(1, 0) });

This works by using GPUQueue.writeBuffer() with a byte offset, so only one element's bytes are sent to the GPU.

What changed:

• src/webgpu/p5.RendererWebGPU.js
  added set(index, value) to StorageBuffer
  added JSDoc and examples
• test/unit/webgpu/p5.RendererWebGPU.js
  added 7 unit tests for float buffers, struct buffers, bounds checks, and type validation

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

[aashu2006]

hii @davepagurek, thanks for waiting!

as there isn’t a way to update a single buffer element without re-uploading the whole buffer, it can get expensive for large cases like 10k particles.

since GPUQueue.writeBuffer() already supports byte offsets, I added a set(index, value) method that computes the element position and writes only that chunk of data.

For struct buffers the offset is index * schema.stride, and for float buffers it’s index * 4.

I reused the existing _packStructArray helper to pack a single struct element (as a one item array), then passed packed.buffer to writeBuffer so the size is handled correctly in bytes.

also added the type checks with clearer errors, plus unit tests for both float and struct buffers including checks that neighbouring elements stay unchanged.

let me know if you would like anything adjusted!